A power capacitor of the above-mentioned kind is built up of a number of sub-capacitors, so-called elements. Each element comprises a plurality of very thin layers of electrodes of thin aluminium foils separated by films of a dielectric material wound into a roll, which is flattened to be able to be optimally stacked into a package. Packages are placed in a capacitor tank filled with an electrically insulating fluid. The elements are connected together into a matrix and connected to two insulated bushings, the number of series-connected and parallel-connected elements, respectively, being determined by the capacitance desired in the capacitor. Usually, a fuse is also connected in series with each element. In addition to elements and fuses, the capacitor also includes special resistors for discharging residual charges.
When two separated, electrically conducting plates are charged with charges of opposite potentials, an electrostatic field is generated. In this field a mechanical force arises between the plates, which attracts the plates towards each other. When the charge is conducted away, the attractive force ceases whereas it again arises if the plates are charged with the reverse polarity. When a charge is applied to the plates at a certain frequency, each time with an alternating potential, an alternating mechanical force therefore arises between the plates. This force gives rise to a mechanical movement of the plates, the frequency of this movement being twice the frequency of the applied alternating charge. For a wound capacitor element consisting of a very large number of layers, the movement of each inner layer is added to the outer layer, whereby the movement in the outer layer may become very large. A drawback with such a capacitor element is, therefore, that it starts oscillating when being loaded, whereby vibrations in the surface layer of the capacitor element cause radiation of unwanted sound or noise.
A common measure for reducing noise from capacitors of the above-mentioned kind, comprising a plurality of elements which when loaded generate vibrations which are transported to the outside of the capacitor tank from where they cause sound radiation, is to screen the capacitors or enclosing them completely or partially. A problem which arises when screening the capacitors is that, since the capacitor often have a considerable potential difference in relation to ground, the screening must be placed at a certain safety distance. A larger distance between the screen and the sound source results in inferior screening effect and, in the case of enclosed capacitors, large casings. This increases both weight and consumption of material. These measures are costly.
Within the audio technique, a capacitor is known, from a patent JP 60-16 739 (1985), which comprises a capacitor element wound from electrodes and plastic films and which is placed in a casing with an inner and an outer layer. The inner layer may be either of elastic or non-elastic material. The outer layer may also either be of elastic or non-elastic material but must be of a kind opposite to that of the inner layer. This capacitor design does not solve the acoustic problem of the capacitor under discussion, which comprises several capacitor elements and which is so large that a layer of the above-mentioned kind would have no damping effect on the sound generation.